<!DOCTYPE HTML>
<html>
<!--
https://bugzilla.mozilla.org/show_bug.cgi?id=1151421
-->
<head>
  <title>Test for Bug 1151421</title>
  <script type="application/javascript" src="/tests/SimpleTest/SimpleTest.js"></script>
  <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css"/>
</head>
<body>
<a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1151421">Mozilla Bug 1151421</a>

<script type="application/javascript">

/** Test for Bug 1151421 **/
SimpleTest.waitForExplicitFinish();

function childLoad() {
  // Spin the event loop so we leave the onload handler.
  SimpleTest.executeSoon(childLoad2);
}

function childLoad2() {
  let cw = iframe.contentWindow;
  let content = cw.document.getElementById("content");

  // Create a function to calculate an invariant.
  let topPlusOffset = function()
  {
    return Math.round(content.getBoundingClientRect().top + cw.pageYOffset);
  }

  let initialTPO = topPlusOffset();

  // Scroll the iframe to various positions, and check the TPO.
  // Scrolling down to the bottom will adjust the page offset by a fractional amount.
  let positions = [-100, 0.17, 0, 1.5, 10.41, 1e6, 12.1];

  // Run some tests with scrollTo() and ensure we have the same invariant after scrolling.
  positions.forEach(function(pos) {
    cw.scrollTo(0, pos);
    is(topPlusOffset(), initialTPO, "Top plus offset should remain invariant across scrolling.");
  });

  positions.reverse().forEach(function(pos) {
    cw.scrollTo(0, pos);
    is(topPlusOffset(), initialTPO, "(reverse) Top plus offset should remain invariant across scrolling.");
  });

  SimpleTest.finish();
}

</script>

<!-- When the iframe loads, it calls childLoad(). -->
<br>
<iframe height='100px' id='iframe' src='file_bug1151421.html'></iframe>

</body>
</html>
